The primary objective of the proposed research is to measure the extent of genetically determined structural heterogeneity within electromorphs (electrophoretic allozymes) of enzymes in house mice, humans, fruit flies, and bacteria in an attempt to estimate the total allelic diveristy at structural gene loci in these organisms. Allelic substitutions that do not alter electrophoretic mobility of polypeptides ("hidden variation") will be detected and characterized primarily by measuring thermostability, but also by assessing electrophoretic response to treatment with thiol reagents. The methodological procedure involves using electrophoresis to reveal amino acid substitutions (affecting electrostatic charge, for the most part) between proteins, calibrating heat-denaturation methods by comparing electromorph pairs, and then screening electromorphs for heterogeneity among individuals at this level of resolution. If heat-denaturation techniques can be calibrated at or near the level of single amino acid substitutions, it will be possible to estimate total allelic diversity at loci. But even if such calibration is not possible, relative amounts of hidden variation between and within electromorphs in various species can be used to test recently developed theories regarding the relationship between effective populatiaon size and the ratio of electromorphs to alleles. The research is designed to yield results important in determining the bases of maintenance of molecular polymorphism in natural populations of mice, humans, and other organisms.